Electrical problem with 99 Tahoe

I have a 99 pursuit package Tahoe and recently the speedometer would start to jump wildly at around 60 MPH. Now the jumping begins around 40-45. When it does this it jumps 10-20 MPH either direction from your actual speed and violently. I thought maybe the speed sensor, or sending unit, something like that. Now the Tahoe also seems to "miss" around around this same speed and when the speedometer is acting up. Also noticed that the headlights will flicker slightly and frequently while this is going on. Just changed the battery and there was no change to the symptoms. Below 40-45 MPH the Tahoe seems to run fine. ..for now! Can anyone help? Thanks!

Welcome to AF. Please tell us what motor, 4wd or 2wd, Old body style or New body style.

If you have a volt meter, what is it doing when this is happening?

Guess that could help some! This Tahoe has the 5.7 L Vortec V8, it is 2WD and not sure on body style, but it has the "old style" sealed beam headlamps. I can see a fluctuation on the voltage with a digital readout, but it fluctuates so fast it is hard to tell. It usually runs around 14.5 VDC. It drops for only a fraction of a second then right back up.

I can see a fluctuation on the voltage with a digital readout, but it fluctuates so fast it is hard to tell. It usually runs around 14.5 VDC. It drops for only a fraction of a second then right back up.

OK. With the volt meter fluctuating at the same time all the other stuff is going on I'd have to say that there is trouble in the Batt feed circuit. Check the positive and negative battery cables at both ends for tightness. If there has been corrosion at the batt connections then I would replace both cables as the corrosion can get to places you can't see.

Another suspected area could be the ign sw and the wiring going to and from it.